The present invention relates to microprocessor based electro-pneumatic type locomotive brake control systems and particularly to a brake assurance circuit for such a locomotive brake control system.
Modern-day locomotive controls, including the locomotive brake control system, incorporate computer technology to reduce hardware and to facilitate adaptation of the system to various customer requirements.
In one such brake control system, a cab-mounted, a handle-operated, brake controller outputs a desired brake command signal to a microprocessor unit, which interprets this brake command signal in terms of a feedback signal indicative of the pressure of air in an equalizing reservoir, and then effects operation of application and release electro-magnetic valves to adjust the equalizing reservoir pressure in accordance with the brake command signal.
A high-capacity pneumatic relay valve device is employed to vary the trainline brake pipe pressure in accordance with variations of the equalizing reservoir pressure, in order to control the railway car brakes. This so-called brake pipe control circuit of the afore-mentioned brake control system is shown and described in U.S. Pat. No. 4,904,027, which is incorporated herein by reference.
The brake control system further includes a locomotive brake cylinder control circuit having electro-pneumatic application and release valves. The locomotive brake cylinder control electro-pneumatic valves are operated by the microprocessor in response to changes in brake pipe pressure initiated by the brake pipe control circuit in accordance with movement of an automatic brake handle of the cab brake controller. Another high-capacity pneumatic relay valve device regulates the pressure in the locomotive brake cylinders according to the pressure output of the locomotive brake cylinder control circuit application and release electro-pneumatic valves.
The electro-pneumatic valves in the brake pipe control circuit and in the locomotive brake cylinder control circuit are arranged to assume a pressure release state, in the event of a power loss at the microprocessor unit. In consequence of such a power loss, therefore, brake pipe pressure is reduced while, concurrently, the locomotive brake cylinder pressure is released. A pneumatic back-up control valve in the locomotive automatic brake control circuit is provided to establish locomotive brake cylinder pressure in response to the aforementioned reduction of brake pipe pressure resulting from such fail-safe operation of the electro-pneumatic valves in the brake pipe control circuit, there being a double check valve to separate the pneumatic backup control valve from the electro-pneumatic valves in the locomotive brake cylinder control circuit.
The pneumatic backup control valve includes a piston valve assembly subject on opposite sides to compressed air in the brake pipe and in a control reservoir. When brake pipe pressure is reduced, the resultant pressure differential forces the piston valve assembly to application position, wherein the control reservoir air supplies the brake cylinder pilot line to establish the locomotive brake pressure until a force balance is restored across the piston valve assembly. In this manner, the piston valve assembly seeks a lap position in which the supply of brake cylinder pressure is terminated at a value corresponding to the brake pipe pressure reduction in effect.
This locomotive brake cylinder pressure may be released independently of the car brakes by means of a quick release valve associated with the pneumatic backup control valve. A pressure signal supplied to the quick release valve, when a quick release switch is actuated, initiates this "bail-off" or quick release function. The brake pipe/control reservoir pressures are communicated via the quick release valve to establish pressure equalization across the piston valve assembly, when the quick release switch is actuated. In this manner, the control reservoir pressure is effectively equalized with the reduced brake pipe pressure, such that spring force acting on the piston valve assembly is effective to force the piston valve assembly to release position and the locomotive brake cylinder pressure is exhausted.
In order to reapply the brake, the quick release switch is deactuated to interrupt communication between the brake pipe and control reservoir pressures via the quick release valve, so that a subsequent reduction of brake pipe pressure is effective to produce a further pressure differential across the piston valve assembly. In response to this further pressure differential, the piston valve assembly will return to application position and reestablish the supply of control reservoir pressure to the brake cylinder pilot line to reapply the locomotive brake.
As previously mentioned, however, brake pipe pressure is reduced to zero in response to a power outage at the microprocessor, due to the fail-safe configuration of the electro-pneumatic valves in the brake pipe control circuit. Following a quick release maneuver, in response to which the locomotive brake cylinder pressure is released, no further reapplication of the brakes can be obtained via the pneumatic backup control valve since brake pipe pressure is essentially zero. It will be appreciated, therefore, that with the electro-pneumatic valves in the locomotive brake cylinder control circuit arranged to release locomotive brake pressure during a microprocessor power failure, and the pneumatic backup control valve disabled, as explained, during a microprocessor power failure, a situation exists in which the train can be moving without the ability to obtain any locomotive braking whatsoever.